Patients in the thyroid carcinoma protocol undergoing follow-up whole body 131I scanning were enrolled in a study of the effect of short-term hypothyroidism on glucose metabolism in the brain. Positron emission tomography with [18F]-fluorodeoxyglucose was used to evaluate total glucose metabolism in 49 cerebral regions. Only one, the thalamus, showed metabolic rates significantly lower than that in the total cerebrum. This may be related to any inadequate relay of sensory input by the thalamic nuclei.